The hand and forearm constitute a complex arrangement of muscles and ligaments that enable a person to perform a multitude of complicated tasks requiring strength and dexterity.
A continuous muscular stress greater than approximately 15% of the maximum stress of a muscle group results in early fatigue of that muscle group with attendant compromise of task effectiveness, increased musculoskeletal disorders, and accident rates. (W. Rohmert, Applied Ergonomics, Vol. 4, pages 91-95, 1973; Butterworth Scientific Ltd.) It is therefore vitally important to develop means to exercise these various hand and forearm muscle groups which are activated in pursuit of everyday activities to improve work quality and recreational pleasures while reducing musculoskeletal injuries.
A number of hand exercisers have been invented that are generally single resistance level devices or whose resistance adjustments are comprised of springs or rubber bands. All of these devices offer a resistance that varies with the movement of the grip, getting progressively more difficult as the muscles are flexed. This progressive resistance can be limiting because the strength needed for squeezing is at a fixed level for any given point during muscle flexion.
Examples of progressive resistance exercisers include U.S. Pat. No. 4,222,560, No. 4,530,496, and No. 5,643,138. They all describe devices that are closed fluid systems, having chambers filled with fluid which have no interaction with atmospheric air. These progressive resistance exercisers employ resistance directly in proportion to the force applied to them and they have no means for adjustment of that resistance.
With these devices, weaker users reach their maximum strength level before they have exercised the full range of motion of their hand which is often so important for such users. Isokinetic resistance, however, or the exertion of muscle force at a constant velocity, allows for different strengths at any given point during muscle flexion because resistance depends on the speed at which the muscle is contracting. In other words, the difference is that when a user is at the extreme end of their muscle contraction, a progressive resistance exerciser is at its most difficult with resistance being the greatest. An isokinetic resistance exerciser, however, is at the same resistance level as throughout the exercise range providing that the speed of the muscle contraction remains constant. Therefore, weaker users can exercise their full range of motion at a rate comfortable to them with isokinetic resistance.
U.S. Pat. No. 4,949,729 and U.S. Pat. No. 5,005,826 are similar to the progressive exercisers but they include bladders that are pre-inflated. These pre-inflated bladders can be set to different levels of pressure before being compressed. Once the pressure level is set, however, the bladders become closed systems, thereby making the devices progressive exercisers with the limitations set forth above.
U.S. Pat. No. 5,338,276 describes a pressure pad in the form of a flexible bladder or bag which comprises a plurality of substantially separate compartments, a pumping means in the form of a pressure bulb or air bulb, and a feedback means in the form of an aneroid dial to permit monitoring or metering of pressure biofeedback transmitted to the pressure pad from the body part of the patient. Also provided is valve means in the form of a regulating screw which may regulate the air flow between the air bulb and aneroid dial. This is similar to a sphygmomanometer, a device used to measure blood pressure. For users to encounter significant resistance in the hand by squeezing the pressure bulb, the pressure pad must first be mostly inflated which, because is has a plurality of substantially separate compartments, may take a number of squeezes of the pressure bulb, or there must be significant pressure against the pressure pad by a body part other than the hand squeezing the pressure bulb. Immediate resistance and feedback is not possible. In addition, the biofeedback is a result of the pressure of that body part on the pressure pad, rather than by the hand squeezing the pressure bulb.
U.S. Pat. No. 3,542,363 departs from the previous patents by employing a deformable bulb for exercising the hand, having air flow control means that allows a rapid intake of air when the bulb expands and resistance to the expulsion of air when the bulb is collapsed. The primary limit of this invention is that it has no means for adjustment. The isokinetic resistance it offers is solely for a single resistance level. If a weaker user wants to exercise the hand over the full range of motion at a high frequency, they cannot do so because the single outlet of air from the bulb remains fixed, so the weaker user must reduce their rate of squeezing which lessens the resistance and therefore the frequency of the squeezing is reduced, contrary to the goal. An outlet that could be adjusted to be bigger would solve this problem. Conversely, stronger users who wish to squeeze at a low frequency with greater resistance cannot do so because their strength forces air out of the outlet too fast, increasing the frequency which is contrary to the goal. An outlet that could be adjusted to be smaller would solve this problem. In addition, because the single outlet of air cannot be closed off, it does not offer progressive resistance as an alternative level of exercise.
There is a need for the present invention because most hand and arm exercisers do not offer the range of strength levels and differing resistances that the present invention offers. Also, many require a closed system that may leak fluid over time. Even the device in the '363 patent is limited in this capacity by not having means for adjustment. In addition, measuring the level of strength being exerted in many of these devices requires a second body part, other than the hand, to accomplish the measurement. The present invention overcomes these limitations by offering both progressive resistance and adjustable isokinetic resistance, as well as means for measuring the strength being exerted solely by the hand and arm, in a single, comfortable device.